1. Field of the Invention
The present invention relates to computerized color image processing of digital color images, and more particularly to systems and techniques for automatic contrast enhancement, automatic color cast removal with contrast enhancement, and combined color cast removal and contrast enhancement where a user can adjust a single control for maximum color cast removal, maximum contrast enhancement, or any intermediate position.
2. Description of the Related Art
In many instances an original color image will include an undesirable color cast or tint. For example, as photographs age they often acquire a yellow cast. In addition, color casts may arise from a variety of circumstances, including the type of lighting or the quality/color sensitivity of photographic film used when taking a photograph.
Conventional techniques have been employed in order to reduce or eliminate color casts in original color images. However, in order to achieve good results, it has generally been necessary to have significant user input. This can be inconvenient in many instances, and can also lead to less than optimal results.
For example, in one conventional technique the user is required to identify the white point in the image, and then the image colors are adjusted accordingly. However, with such a technique, different users might disagree about which point in the image is the white point. Accordingly, the image processing parameters will generally be based upon a subjective determination, which might not always be desirable. Moreover, a user's determination of the white point might be skewed based on the color makeup of the image. For example, a point might erroneously appear to be the white point only because surrounding colors interfere with the user's perception of the point's true color. Finally, selecting the white point in each image can be very tedious and burdensome, particularly when many images are to be processed, and generally will be impractical if one wishes to process frame images on a real time basis.
Another common defect of color images is poor contrast. This problem can obscure image information, as well as negatively affect the general appearance of the image. Although conventional techniques exist for enhancing contrast, such conventional techniques generally do not provide optimal results in many situations. For example, many conventional contrast enhancement techniques do not provide acceptable results when the original image has both very dark and very bright pixels.
Moreover, it is often desirable to allow a user to have some control over the amount of contrast enhancement and color cast removal performed on the original image. For instance, a fully automatic color cast removal technique probably will be incapable of determining when a user subjectively wants the image to have a color cast, such as when a user has taken a photograph through a colored filter or when a user intentionally wishes to mimic an old and yellowed photograph. Similarly, a user might, for aesthetic reasons, want to vary the amount of color contrast in an image. Conventionally, however, in order to permit a user to adjust both the amount of contrast enhancement and the degree of color cast removal, at least two controls have been provided (one for color cast removal and one for contrast enhancement). Accordingly, simultaneous adjustment of both color cast removal and contrast enhancement using conventional techniques has not heretofore been possible.